Needs exist for sensitive low-cost optical instruments which can detect trace levels of gases or volatiles in solution or in air.
The rapid increase in world seafood production has placed a competitive burden on the U.S. fishing and fish-farming industries, and has accelerated the development of high-density closed-cycle aquaculture (HDCCA) unit operations to provide seafood products at a competitive unit cost. Quality control in HDCCA requires rapid analysis of critical chemical concentrations of dissolved-oxygen and dissolved-ammonia in recycled water streams; however, no single sensor or group of sensors is currently available to provide timely information on water quality throughout HDCCA facility.
The use of aquaculture for production of seafood products has increased dramatically from one million metric tons annually in 1966 to eleven million metric tons in 1987. Among aquacultural systems, closed cycle facilities are attracting great interest as a result of the increased productivity and decreased environmental damage associated with water reuse.
The effective operation of closed cycle aquaculture facilities requires the development of in-process monitors to alert service personnel to a reduction in dissolved-oxygen or increased ammonia or nitrite that may result in decreased product quality, reduced yield or fish kills.
It has been shown that the buildup of organic carbon and inorganic substances including ammonia, nitrates and phosphates may be implicated in reduced fish quality. The benefits of continuous monitoring of those organic and inorganic materials are found in increased process density afforded by rapid response to anoxic conditions, reduced product loss from toxic material buildup or disease and improved competitiveness related to product quality. Although several techniques have been considered to provide those important measurements, no single technique has demonstrated sufficient versatility to address each of those sensor requirements.
HDCCA operations have become increasingly important due to increased demand for seafood products, because of health consciousness and decreasing availability due to aggressive harvesting practices. Aquaculture systems based on recirculating water have become popular due mainly to reduced water supply demand as well as wastewater discharge. Such systems can also solve many of the seasonal and site-limiting problems of pond systems by being placed inside buildings, where the entire fish culture environment can be controlled and managed.
In-process monitoring of certain chemicals in HDCCA systems becomes important because such information can be used in automatic control systems and by human operators. Continuous monitoring of environmental conditions is important because of effects on fish health, feed utilization, growth rates, stocking densities, carrying capacities and waste management. The environmental variables in aquaculture processes include temperature, dissolved-oxygen, pH, ammonia, nitrite, nitrate, suspended solids, turbidity, salinity and water flow rates. Needs exist for sensors for monitoring dissolved-oxygen, dissolved ammonia and nitrate/nitrite in HDCCA waters.
Dissolved-oxygen concentration is perhaps one of the most critical factors for healthy fish growth. Fish rely on the oxygen dissolved in water to support their metabolism. A decrease in dissolved-oxygen level below a certain critical value will result in fish kill in a matter of several minutes. Therefore, it is critically important to be able to monitor dissolved-oxygen in HDCCA facilities. The ability of water to carry oxygen depends on temperature, pressure and dissolved salts.
Various species of fish can tolerate different levels of dissolved-oxygen concentration. For example, salmonid need a minimum level of 5.0-5.5 mg/L of dissolved-oxygen for healthy growth, while carp, catfish and tilapia can withstand dissolved-oxygen levels of below 2 mg/L for short periods of time.
Current oxygen sensors are oxygen-selective membrane electrodes of the polarographic or galvanic type, which consist of two metal electrodes in contact with electrolyte and separated from the test solution by a gas permeable membrane. To measure dissolved-oxygen concentration, the oxygen permeates a plastic membrane (polyethylene or fluorocarbon), reacts with the reactive electrolyte and is measured using the metal electrodes. With that type of sensor, three problems arise when used in an aquaculture process.
Firstly, the sensor consumes a portion of the oxygen surrounding the sensor head, thereby requiring water to be recirculated to the surface of the sensor head. Secondly, bacteria grow on the gas-permeable membrane, accumulate and prevent oxygen penetration. As a result, cleaning of the membrane is needed at least once a day. Thirdly, the signal from that type of sensor drifts with time. In addition, the sensor head is bulky and prohibitively expensive.
Silicone polymer is the most commonly used solid support for optical oxygen indicators, because of its oxygen permeability and also because it does not quench the fluorescence of most of the oxygen indicators. Most work on optical sensors for dissolved-oxygen concentration has been based on the water soluble indicator ruthenium(II) tris(bipyridyl) (Ru(bpy).sub.3.sup.2+) dichloride in silicone. For example, complexes of ruthenium ions with bipyridine or phenanthroline ligands may be used to develop an oxygen sensor based on fluorescence quenching. Sensors based on ruthenium with phenanthroline ligands have been found to be less sensitive to changes in the embedding media than those based on the bipyridyl ligand.
Rapid evaluation of HDCCA water quality also requires data on the concentration of ammonia-nitrogen. One of the most toxic chemicals to fish is the un-ionized form of ammonia-nitrogen in the main excretory products of fish, and is most likely to accumulate over time in HDCCA systems. In water, the ammonia molecule exists in equilibrium with its ionized form as follows: ##STR1##
The ammonium ion (NH.sub.4.sup.+) is fairly innocuous to fish, whereas free ammonia (NH.sub.3) is highly toxic, and a level of 0.02 mg/L is generally regarded as the maximum acceptable limit for healthy fish.
Currently, there is no in-process ammonia sensor for use in aquaculture processes although several electrochemical instruments are available for laboratory analysis. Total ammonia-nitrogen concentration is determined by a wet chemistry process, such as the Nessler method, which has a sensitivity of 20 .mu.g/L using a spectrophotometer. The ammonia-selective electrode has drawbacks of not having sufficient sensitivity and being subject to biofouling and interference from dissolved ions, such as mercury and silver.
Optical absorption-based sensors have been widely used for ammonia detection. For example, bromothymol blue, oxazine 750 perchlorate, and bromocresol purple have been used as absorbing indicators for ammonia detection. Also a fluorescent dye, acridine orange, has been used as an indicator for ammonia. Although the instrumentation of those optical techniques to detect ammonia is different, the basic principle remains the same.
The principle underlying those detection schemes is based on the change of optical signal as ammonia gas traverses a permeable membrane and reacts with a pH sensitive indicator. The pH indicator changes the absorption spectrum, thereby indicating the concentration of ammonia. This scheme is feasible because ammonia is an alkaline gas and only several other gases, such as dimethylamine, trimethylamine and hydrazine hydrate (rarely found to have enough concentration to interfere with detection), are more basic than ammonia. The reaction of ammonia, NH.sub.3, with a pH-sensitive dye molecule, such as bromocresol purple (BCP) and bromothymol blue (BTB), in the aqueous media is summarized as follows: ##STR2##
Toxicity of nitrite to fish is also of concern in aquaculture systems, although fish are much less sensitive to nitrite than to dissolved-oxygen and un-ionized ammonia. Nitrite is known to cause conversion of hemoglobin to methhemoglobin, which is incapable of binding and transporting oxygen. The level of nitrite sensitivity varies widely across various fish species. For example, toxication is observed for rainbow trout at 22 mg/L, but at 453 mg/L for large mouth bass. Current methods of measuring nitrite concentration in aquaculture processes involve the use of diazotization, a method involving the formation of reddish purple azodye produced at a pH of 2.0 to 2.5 by coupling diazotized sulfanilic acid with N-(1-naphthyl)-ethylenediamine dihydrochloride. With the use of a spectrophotometer, that method determines nitrite concentrations as low as 1 mg/L.
Optical methods for detection of nitrite have been evaluated. Most of those methods are based on the four-step Griess reaction resulting in the formation of a diazo dye. Nitrite ions react with sulfanilamide to form diazonium cations in acidic solution. That is coupled to N-(1-naphthyl) ethylenediamine (NED) to form the dye. The dye has absorption maxima at wavelengths of 350 nm and 540 nm and is suitable for use with green emitting helium-neon laser excitation. However, that analytical method is not compatible with optical sensors for continuous monitoring because of the number of steps involved and the requirement for an acidic environment to form the diazonium cations.
Development of dissolved-oxygen and dissolved-ammonia sensors is required, because dissolved-oxygen in HDCCA systems can fluctuate rapidly, and lack of oxygen for several minutes can result in fish kill and because extremely low concentrations of dissolved-ammonia can be deleterious to fish survival and taste.